We have studied van der Waals contacts of the carotenoid rhodopin glucoside (RG) with the bacteriochlorophyll pigments absorbing at 800 nm (B800) in the crystal structure of Rhodopseudomonas acidophila, and the hydrogen positions were determined from quantum chemical calculations at the Hartree--Fock (6-31G) level. We have found strong evidence for hydrogen bonding between the B800 BChl and the RG from neighboring protomer units. The binding energy was estimated to be about 2 kcal/mol (700 cm(-1)). CI-singles approach and time-dependent density functional theory calculations of the B800--RG dimer indicate a red-shift (ca 2 nm) of the B800 Q(y) transition, along with a substantial increase of its oscillator strength, probably due to the hydrogen bonding.
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[Show abstract][Hide abstract] ABSTRACT: Carotenoids play the dual function of light harvesting and photoprotection in photosynthetic organisms. Despite their functional importance, the molecular basis for binding of carotenoids in the photosynthetic proteins is poorly understood. We have discovered that all carotenoids are surrounded either by aromatic residues or by chlorophylls in all known crystal structures of the photosynthetic pigment-protein complexes. The intermolecular pi-pi stacking interactions between carotenoids and the surrounding aromatic residues in the light-harvesting complex II (LH-II) of Rhodospirillum molischianum were analyzed by high level ab initio electronic structure calculations. Intermolecular interaction energies were calculated with the second-order Møller-Plesset perturbation method (MP2) using the modified 6-31G*(0.25) basis set with diffuse d-polarization by Hobza and co-workers. The MP2/6-31G*(0.25) calculations yield a total stabilization energy of -15.66 kcal/mol between the carotenoid molecule and the four surrounding aromatic residues (alpha-Trp-23, beta-Phe-20, beta-Phe-24, beta-Phe-27). It is thus concluded that pi-pi stacking interactions between carotenoids and the aromatic residues play an essential role in binding carotenoids in the LH-II complex of Rhodospirillum molischianum. The physical nature of the pi-pi stacking interactions was further analyzed, and the dispersion interactions were found to be the dominant intermolecular attraction force. There is also a substantial electrostatic contribution to the overall intermolecular stabilization energy.
Journal of the American Chemical Society 08/2002; 124(28):8445-51. DOI:10.1021/ja025618i · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Density Functional Theory (DFT) is an effective and popular approach to the quantum many-body problem. DFT is exact but, in practice, the exchange-correlation contribution to the total energy must be approximated. Because of the demand for higher accuracy in applications, the modeling and understanding of this exchange-correlation density functional is an important area of research and is the focus of this thesis. First, a DFT is developed for a one-dimensional contact interaction. Then, the focus returns to the Coulomb-interacting problem. Uniform density scaling relationships are generalized to spin-scaling relationships. The adiabatic connection is considered at both strong and weak interaction limits. In the strong limit, the strictly-correlated electron hypothesis is tested. In the weak limit, the accuracy of exact-exchange DFT as applied to large gap solids is examined.